Control of automatic abrading machines



Nov. 12, 1958 M. R. ESTABROOK 3,410,028

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EXPANDER ROD FORCE 8 0 no 20 so 40 so so 70 so so MOTOR VOLTAGE vou'sUnited States Patent 3,410,028 CONTROL OF AUTOMATIC ABRADING MACHINESMark R. Estahrook, Rockford, Ill., assignor to Barnes Drill Co.,Rockford, Ill., a corporation of Illinois Filed Mar. 30, 1965, Ser. No.443,875 12 Claims. (Cl. 51-165) ABSTRACT OF THE DISCLOSURE An automatichoning machine having a reciprocating rotary tool of conventional formthat is progressively expanded within a work bore to feed the honingstones into the bore wall, thereby removing surface roughness andenlarging the bore to a predetermined size. The expansion mechanism isdriven by a variable speed and torque electric motor having an armaturethat draws current through a tungsten lamp bulb filament so that theresistance value of the filament increases with the current level tomodulate the motor torque and honing force in accordance with the loadon the tool. The force range of the motor is varied either by a rheostatin parallel with the lamp or by a voltage divider for adjusting thefield strength of the motor.

This invention relates to the control of automatic abrading machines:and is particularly adapted for use in a honing machine for enlargingand finishing work bores by reciprocating and rotating an expandablehoning tool within the bore while gradually expanding the tool to pressthe abrasive elements thereon against the bore wall. More particularly,the invention relates to the control of an electric motor driving themechanism for feeding abrasive elements into a rough work surface at acontrolled rate that determines the rate of removal of stock from thesurface.

In the honing operation, efficient action of the tool is dependent uponthe pressure with which the abrasive stones of the tool are forcedagainst the bore wall by the hone expansion mechanism. Optimum honeperformance depends upon the proper correlation of motor torque and feedrate with the resistance to expansion offered by the bore wall. If thetool is expanded too rapidly, the abrasive stones are subjected toexcessive pressure and destructive strains which tear the grit out of orcrack the stones, thereby materially reducing the useful life of thestones. If, on the other hand, the rate of expansion is too slow, thelength of the honing cycle is increased and, in addition, the efiiciencyof the stones may be impaired by glazing, that is, either loading of theexposed surfaces t of the stones with worn-away material or dulling ofthe exposed abrasive particles at the surfaces of the stones. Thus, bothtoo slow and too rapid expansion rates materially increase productioncosts.

Control of hone expansion to produce optimum hone performance iscomplicated by the nature of the work surfaces to be honed. At thebeginning of a honing operation, the bore wall is comparatively roughand irregular and the hone operates primarily on ridges and high spotsleft on the wall by the previous operation. These can be worn away at arelatively rapid rate. As honing progresses, however, the stonesencounter a progressively increasing surface area and correspondinglyincreasing resistance to expansion of the tool.

The general object of the present invention is to reduce the length ofthe honing cycle and increase the life of abrasive stones in service useby maintaining optimum honing pressure throughout the honing cycle.

Another object is to sense the changing resistance offered by the worksurface throughout the cycle and vary the feed rate automatically andprogressively in response to changes in such resistance.

A more detailed object is to sense the resistance by measuring thecurrent drawn by the electric motor driving the feed mechanism, :and toregulate the speed and torque of the motor automatically in response tovariations in the motor current to obtain an expansion rate and motortorque for producing optimum honing pressure for the prevailingcondition of the work bore, that is, a high rate of initial expansionand progressively slower expansion with controlled torque as the workresistance increases.

A further object is to provide flexibility of adjustment of the range ofexpansion rates, motor torque, and the resulting honing pressuresobtained during the honing cycle to adapt the cycle for differentanticipated honing conditions.

Still another object is to obtain optimum honing effectivenessthroughout the honing cycle for increased stone life, reduced cycletime, and generally improved hone performance.

Other objects and advantages of the invention will become apparent fromthe following detailed description taken in connection with theaccompanying drawings, in which FIGURE 1 is a fragmentary frontelevational view of an automatic honing machine embodying the novelfeatures of the present invention.

FIG. 2 is a schematic view showing the feed mechanism.

FIG. 3 is a diagram of the important parts of the feed control.

FIG. 4 is a chart showing the changes in the resistance of the filamentof a lamp bulb, rated at 50 watts at 250 volts, with changes in thecurrent through the filament.

FIG. 5 is a diagrammatic view similar to FIG. 3 but showing a modifiedform of the invention.

FIG. 6 is a diagrammatic view showing still another form of theinvention.

FIG. 7 is a chart illustrating the effect of changes in the rheostatresistance on the feed motor voltage and expander rod force ranges.

As shown in the drawings for purposes of illustration, the invention isincorporated in a power actuated mechanism for expanding a tool 10(FIG. 1) for honing the bore wall 11 of :a workpiece 12 by reciprocationof the tool along the bore and simultaneous rotation of the tool withabrasive elements 13 thereon pressed against the bore wall. In thisinstance, the tool is carried on a shank 14 on the lower end of aspindle 15 journaled on a head 17 slidable back and forth on verticalguide rods 18 supported on the machine frame. The spindle :and the shankare connected by a universal joint 19 and are rotated by an electricmotor (not shown) connected to the spindle by gearing 20. Raising andlowering of the head along the guide rods 18 to reciprocate the toolwithin the bore is effected by a suitable actuator (not shown) in a wellknown manner.

The honing tool 10 is of a conventional type comprising a series ofelongated abrasive sticks or stones 13 of generally rectangularcross-section disposed in and projecting outwardly through elongatedslots angularly spaced around a hollow cylindrical body 21 on the lowerend of the shank 14 with followers 22 on the inner sides of the stonesengaging a conical cam 23 within the body. The cam shown herein is faston an axially movable rod 24 which extends upwardly through the toolshank and the spidnle 15 and is part of the mechanism for expanding thetool within the bore, the rod being formed in two separate sections topermit pivotal motion at the joint 19. As the rod is moved downwardlyrelative to the shank and the stones, the latter are ca-mmed radiallyoutwardly :at a rate determined by the slope of the cam and the rate fendwise movement of the rod, and are pressed against the bore wall 11with a force proportional to the force exerted on the rod.

Feeding of the stones 13 outwardly into the bore wall 11 during honingis effected by an electric motor 25 driving the expansion mechanism andoperable to feed the expander rod 24 downwardly at a controlled raterelative to the tool. While the connection between the feed motor andthe expander rod may take various forms, herein a worm 26 (FIG. 2) onthe motor shaft meshes with a worm wheel 27 on one end of anintermediate shaft 28 driving another shaft 29 through worm gearing 30.During the honing cycle, this shaft is coupled by a clutch 31 to a driveshaft 32 whose rotary motion is converted to endwise motion of theexpander rod 24 by a worm 33 meshing with a nut 34 threaded onto theupper end of the expander rod and mounted in the head for rotation inthe position shown in FIG. 2. Thus, rotation of the nut in one directionmoves the expander rod downwardly through the spindle and the shank toexpand the tool, while rotation of the nut in the opposite directiondraws the rod upwardly to permit contraction of the tool.

For rapid expansion and contraction of the tool at the beginning and endof the honing cycle, a reversible electric motor 35 is geared at 37directly to the drive shaft 32 to rotate the latter at a rate many timesfaster than that obtained with the feed motor. During operation of thetraverse motor 35, the clutch 31 is deactivated to disconnect the feedmotor and the drive shaft. When the traverse motor has expanded the toolsufficiently to bring the stones 13 close to or into engagement with thebore wall 11, it is deenergized and the feed clutch is engaged toinitiate slower feeding of the stones. Control of termination of rapidhone expansion may be accomplished in various ways, for example, by adevice of the type disclosed in Johnson Patent No. 2,819,566. Duringhoning, the deenergized traverse motor turns idly with the drive shaftas the latter is rotated at a controlled and relatively slow rate indirection to move the expander rod downwardly through the tool shank andfeed the stones radially outwardly at a rate which determines the rateof expansion of the tool.

In accordance with the present invention, the speed and torque of theelectric feed motor 25 are controlled in an extremely simple andeffective manner throughout the honing cycle to vary the feed rate andstone pressure against the work automatically in response to variationsin the resistance encountered by the stones 13 thereby to produce theoptimum feed rate and honing pressure throughout the cycle. In this way,stone life is materially increased and the length of the honing cycle isreduced.

To these ends, the motor armature 38 is energized from a voltage source39 through a series resistor 40 having an impedance or resistance thatincreases and decreases sharply, that is, to a high degree, withincreases and decreases in the current flowing through the resistor.With this arrangement, the progressively increasing resistance toexpansion of the tool 10 during the honing cycle automatically andprogressively reduces the tool feed rate and modulates the torqueexerted by the feed motor, thus regulating the honing pressurethroughout the cycle.

To facilitate an understanding of the operation of the invention, itshould be pointed out that the amount of cur rent drawn by the armature38 in operation is dependent upon the difference between the terminalvoltage impressed on the armature and the back voltage it generates.Back voltage, in turn, is determined by the speed of rotation of thearmature. Accordingly, when a constant voltage is impressed on the motorarmature and field 41, the current drawn increases and decreases as themotor speed decreases and increases. Moreover, the motor speed variesfrom a maximum speed with no load on the motor to zero speed (a stalledcondition) when the load is greater than the motor can move. Thus, thecurrent drawn by the motor varies from a relatively small value when themotor is running under no-load conditions and the back voltage is nearlyequal to the terminal voltage, to a relatively large value when themotor is running very slowly, the back voltage then being near zero invalue. In addition, it should be remembered that the torque exerted bythe motor is directly proportional to and increases with the currentdrawn by the motor.

With these fundamentals in mind, it will be seen that the current drawnby the feed motor 25 through the resistor 40 increases gradually as thehoning tool 10 wears away the rough surface metal of the bore wall 11and encounters progressively increasing resistance to expansion of thetool. Since the current through the resistor is the same as that in themotor, and the resistance value of the resistor increases sharply withincreases in current, the resistance in series with the armature 38rises as the work load increases. Thus, the resistor senses theincreases in the work resistance by measuring the current drawn by thefeed motor.

The immediate effect of the increasing resistance is to reduce theterminal voltage of the motor armature 38 by an amount proportional tothe resistance increase. It is fundamental, of course, that a reductionin the armature voltage correspondingly reduces the speed of the motor,so the resistance increase further reduces the motor speed and the toolfeed rate. In addition, it will be evident that the reduction in theterminal voltage reduces the difference between the terminal voltage andback voltage generated by the armature at the prevailing motor speed,and thereby reduces the current drawn and the torque exerted at thatspeed. Thus, as the motor speed is gradually reduced by theprogressively increasing work load, the resistor 40 reduces the terminalvoltage of the armature and thereby further reduces the motor speedwhile simultaneously controlling and modulating the increase in currentflow and the resulting torque exerted. Accordingly, the resistorconstitutes a load-responsive speed and torque controller for the motor.

In putting the invention into practice, advantage is taken of theWell-known fact that the tungsten filaments of standard incandescentlamp bulbs display the desired resistance characteristics. Suchfilaments have positive temperature coetficients of resistance, that is,a resistance value that increases with the temperature of the filament.The temperature of the filament, in turn, varies over a wide range inaccordance with changes in the current through the filament due to theheating of the filament by energy conversion according to the well-knownrelationship W=I R, where W is the rate of heat energy dissipation inwatts, I is the current in amperes, and R is the resistance in ohms. Theline 36 in FIG. 4 illustrates the rate of rise of filament resistancewith changes in filament current witth a lamp bulb rated at 50 watts at250 volts.

In this instance, the feed motor is a 1/ 150 horsepower shunt-wound DC.motor with its armature 38 and field 41 connected across power lines L1and L3 having input terminals connected to a volt direct-current voltagesource at 39, and with an incandescent lamp 42 in series with thearmature on the positive side of the latter as shown in FIG. 3. To begina honing cycle, the tool 10 first is expanded by the traverse motor 35to bring the stones 13 close to the bore wall 11, the traverse motor isdeenergized, and the clutch 31 is engaged to couple the feed motor 25 tothe expander rod drive shaft 32. The feed motor is started by closing aswitch 43 (FIG. 3) to complete the circuits to the motor. Thus, currentflows through the lamp filament 40 to the feed motor armature 38 andbegins heating the filament as the motor starts. Since the load on themotor is relatively small, the motor almost immediately attains arelatively high speed and feeds the stones outwardly at a coarse feedrate until the stones engage the bore wall.

With the feed motor running rapidly, the back voltage generated by thearmature 38 is nearly equal to the terminal voltage of the armature, andthe current drawn by the motor through the filament 40 is small. At thesame time, the current flowing through the filament rapidly heats thelatter to a temperature correlated with the motor current. It will beappreciated that the initial incerase in the filament temperature andresistance reduces the terminal voltage of the armature. If the workload on the motor remained constant, however, the motor speed, motorcurrent, filament temperature and filament resistance would attainconstant and interrelated equilibrium values.

As the stones 13 engage the bore wall 11, they are fed outwardly at arelatively rapid rate to remove the surface roughness from the wall. Atthe same time, the resistance to feeding of the stones begins toincrease and the increasing work load begins to reduce the speed of thefeed motor 25 so that the back voltage generated by the motor begins todecrease and the motor current begins to increase. This increase incurrent increases the heat produced by the filament 40, correspondinglyraising its temperature and resistance after a very slight thermaldelay. The increase in filament resistance increases the voltage dropacross the fialment and, therefore, correspondingly reduces the terminalvoltage of the armature, further reducing the speed of the motor and therate of expansion of the tool.

As the honing cycle progresses and the stones encounter a progressivelyincreasing surface area, the resistance to expansion gradually increasesand the motor draws more and more current thereby gradually increasingthe filament temperature and resistance, and the terminal voltage of themotor gradually is reduced. This voltage reduction further reduces thespeed of the motor and controls the rate of increase in the motorcurrent and the resulting torque exerted on the push rod 24 and theexpanding force exerted on the stones 13.

When all the ridges, high spots and other surface roughness have beenremoved and the stones 13 encounter base metal, that is, the metal of atrue and substantially smooth cylindrical wall, the remainder of thehoning operation is devoted to the enlargement of the bore to thedesired size. During this portion of the operation, the resistance tofeeding of the stones into the work remains substantially constant.Thus, the load on the feed motor 25 remains constant and the systembecomes stabilized with the motor operating relatively slowly anddrawing current at a relatively high but constant rate that produces aconstant filament temperature and resistance value. Accordingly, thestones continue to be fed relatively slowly and at a fine feed rate intothe bore wall 11 to enlarge the bore until it attains the desired size.The equilibrium values of the motor speed and torque, of course,determine the final expansion rate and honing pressure exerted by thestones.

Optimum honing pressure is that pressure which will wear away the metalof the bore wall 11 at the fastest rate without excessive wear on theabrasive stones 13. This pressure will vary in service use with the typeof abrasive stones being used and the hardness of the metal beingremoved, and the motor torque and push rod force required to produce thedesired pressure will vary with the active area of the stones and thediameter of the bore being honed. Since the torque of the motor 25 isproportional to the current drawn by the armature 38 when the fieldstrength is constant, the torque exerted by the motor is limited by thevalue of the resistance in series with the armature.

While the selected starting voltage and the range of voltage andresistance values obtained during a honing cycle may be changed by usinglamp bulbs 42 of different ratings, more convenient and flexibleadjustment of the motor torque and the feed rate is accomplished bymeans of a rheostat 44 (FIG. 5) connected in parallel with the lampfilament to increase and decrease the equivalent resistance of thefilament-rheostat as the rheostat resistance is adjusted upwardly ordownwardly. In the illustrative control, the rheostat resistancepreferably is adjustable up to 5000 ohms and is used with a lamp bulbhaving a SO-Watt rating at 250 volts.

The setting of the rheostat 44 determines the equivalent resistance ofthe parallel combination of the filamentrheostat at any given filamenttemperature and thus determines the armature voltage and the motorcurrent and torque at any given point in the honing cycle. Higherrehostat settings result in lower torque values both initially and ashoning progresses, while lower settings produce higher motor speeds andtorques with correspondingly greater honing pressures. The effect ofdifferent representative settings on the relative values of motorvoltage and expander rod force with the 50 watt bulb is illustrated inFIG. 7. It will be seen that the addition of a rheostat in parallel withthe automatically adjustable resistor 42 provides a family of curvesfrom which the operator may select the honing rate most favorable for aparticular workpiece.

With the rehostat setting determined by experience to be best suited forthe workpiece 12 and the type of stones 13, the motor torque developedin the equilibrium condition will maintain optimum honing pressure toWear away the base metal of the bore wall 1 without excessive stonewear. A voltmeter 45 preferably is connected across the motor armature38 to indicate the varying armature voltage during the cycle. Since thisvoltage is a direct indication of the prevailing feed rate and motortorque, the tool performance with a selected rheostat setting isindicated by the voltmeter. If the motor voltage does not decline at arapid enough rate toward the minimum voltage expected for the finalportion of the cycle (to a range of 5-10 volts with the illustrativecontrol), this is an indication that the selected rheostat setting isnot high enough and the feed rate is too rapid and the torque is toohigh. If the voltage drops too fast, this is an indication that therheostat resistance is too high and the torque developed is notsufficiently high for the particular Workpiece. Experience with thecontrol makes it possible for the machine operator to evaluate thehoning performance and adjust the rheostat for optimum performance withvarious types of workpieces.

Under production honing conditions, it has been established that theforegoing control can increase stone life by as much as three timeswhile reducing the length of the honing cycle to one-third, as comparedto stone life and cycle time on the same machine equipped with aconventional, constant-speed feed mechanism. The reduction in cycle timeis attributed to two things. First, the faster initial expansion rate isbetter suited to the condition of bores at the beginning of the cycleand removes ridges and high spots from the bore walls more rapidly thanin prior machines. In addition, it is believed that the automaticallyvariable feed rate dictated by the condition of the bore wallssubstantially reduces or eliminates glazing of the stones 13 andaccordingly increases their cutting efficiency. The increased stone lifeprobably is related to the increased cutting efficiency and reduction inheating of the stones in service use, as well as to the precise controlof the honong pressure and avoidance of excessive pressure and wear nearthe end of the cycle when an extremely slow feed rate maintainsrelatively high honing pressure.

In machines that are to be used to hone relatively large bores, it maybe desirable to provide a larger feed motor, for example, a horsepowermotor as shown at 47 in FIG. 6. If such a machine also is to be used tohone relatively small bores, starting difficulties can develop when thearmature rheostat is adjusted for high resistance to lower the speed andtorque range of the larger motor. The modified control illustrated inFIG. 6 is designed for such machines. Instead of controlling thearmature voltage range, a voltage divider 48 is provided for adjustingthe field strength. In this way, the motor torque may be reduced whilemaintaining reasonably high starting speeds. In all other importantrespects, the alternate control is the same as the preferred form.

From the foregoing, it will be seen that the present invention controlsthe feed rate of the abrasive elements 13 and the torque exerted by thefeed motor to maintain optimum working pressure throughout the abradingoperation. The motor first feeds the stones into the work surface at arelatively rapid rate for rapid abrading of the rough surface metalthereby avoiding prolonged cycle time. As the work resistance increases,however, the feed rate is reduced automatically and progressively at arate dictated by the condition of the work surface itself, and the rateof increase in motor torque is controlled to prevent the application ofexcessive pressure. Moreover, the range of expansion rates and motortorques throughout the cycle may be adjusted for different anticipatedhoning conditions. The result i an automatic control that materiallyincreases stone life and reduces the length of the honing cyclerequired.

Amplified theoretical analysis From an inspection of the chart shown inFIG. 4, it will be seen that the filament is a resistor having acurrent-versus-resistance characteristic with a positive slope less than90 degrees, showing that the resistance increases with increases in thecurrent fiow rate. In the lamp filament, of course, this change is dueto the positive temperature coefiicient of the filament and the heatingof the filament to materially different temperatures by differentcurrent rates.

From another viewpoint, it will be evident that the lamp filament is aso-called non-linear resistor (in terms of current-versus-voltagecharacteristics), producing a voltage drop in the armature circuit thatincreases at a progressively increasing rate, whereas the increase involtage drop in linear resistors is at a fixed rate. This characteristicof the resistor is believed to accomplish the desired motor control inaccordance with the progressive increase in workload encountered by thetool, controlling the rate of torque increase to suit the changingcondition of the work.

It should also be noted that the reduction gearing of the expansionmechanism shown in FIG. 2 elfects a very substantial reduction betweenthe feed motor 25 and the push rod 24, and the result of this is asmoothing out of the response of the control by the resistor 40, as wellas enabling the use of standard lamp bulb resistors to control theapplication of substantial working forces without relatively complicatedcontrol modifications. Through this reduction gearing, a change ofseveral pounds of push rod force is reflected in the armature current asa small change on the order of one milliampere which changes thefilament voltage drop only a slight amount.

I claim as my invention:

1. In a honing machine, the combination of, an expandable honing toolhaving radially movable honing elements, feed mechanism for expandingsaid tool by feeding said element outwardly into engagement with a borewall to be honed, a shunt-wound direct-current motor driving said feedmechanism to expand said tool at different rates determined by the speedof operation and torque of said motor, and a control for said motorincluding input terminals adapted for connection to a direct currentvoltage source, an incandescent lamp bulb hav ing a tungsten filamentconnected in series with the armature of the motor between saidterminals to reduce the armature voltage progressively with increases inthe resistance to tool expansion, and a rheostat connected in parallelwith said filament for selectively adjusting the equivalent resistanceof the filament-rheostat combination at any given temperature of thefilament thereby to select the maximum honing pressure produced by saidmotor.

2. In a honing machine, the combination of, an expandable honing toolhaving radially movable honing elements, feed mechanism includingreduction gearing for expanding said tool by feeding said elementsoutwardly into engagement with a bore wall to be honed, a shunt-wounddried-current motor driving said feed mechanism to expand said toolthrough said gearing at different rates determined by the speed ofoperation and torque of said motor, and a control for said motorincluding a direct current voltage source and an incandescent lamp bulbhaving a tungsten filament connected in series with the armature of themotor to reduce the armature voltage progressively with increases in theresistance to tool expansion and the resulting increases in the currentdrawn by the motor thereby to control the feed rate and torque of saidmotor.

3. The combination defined in claim 2 further including means forselectively adjusting the field strength of said motor and therebyadjusting the honing pressure produced by the motor.

4. In an abrading machine, the combination of, a tool having a movableabrading element, mechanism for feeding said element into engagementwith a work surface, a variable speed and torque electric motor drivingsaid feed ing mechanism to feed said element at different ratesdetermined by the speed of operation of said motor, and a control forsaid motor including an incandescent lamp bulb having a tungstenfilament connected in series with the armature of said motor whereby theterminal voltage of said motor is reduced progressively as theresistance to feeding of said element increases, and a rheostatconnected in parallel with said filament for selectively adjusting theequivalent resistance of the filament rheostat combination at any giventemperature of the filament thereby to select a range of motor speed andtorque developed as a surface is abraded.

5. In an abrading machine, the combination of, a tool having a movableabrading element, mechanism for feeding said element into engagementwith a work surface, a variable speed and torque electric motor drivingsaid feeding mechanism to feed said element at different ratesdetermined by the speed of operation of said motor, and a control forsaid motor including an incandescent lamp bulb having a tungstenfilament connected in series with the armature of the motor thereby tocontrol the speed and torque of said motor in response to progressiveincreases in the resistance to feeding of said element.

6. The method of feeding an abrasive element into a rough work surfacewith a variable speed and torque elec tric motor and feed mechanismdriven .by said motor, said method comprising the steps of, initiallyenergizing said motor to run at a relatively rapid speed with a selectedarmature 'voltage thereby to feed the element at a coarse feed rate,continuously sensing the current drawn by the motor armature as theelement is fed into the work surface, and reducing the armature voltageprogressively as the armature current increases and directly in responseto each sensed increase in such current thereby progressively reducingthe motor speed and controlling the motor torque throughout the feedingof said element.

7. The method of feeding a machine tool element into a workpiece with avariable speed and torque feed mechanism, said method comprising thesteps of, initially operating said feed mechanism to run at a firstpreselected no-load rate to feed said element into the workpiece at arelatively rapid rate, sensing the increase in the resistance to feedingof said element as the latter engages the workpiece and continuouslysensing variations in such resistance while the element is being fedthrough the workpiece, reducing the feed rate and increasing the torqueoutput of said feed mechanism in response to all sensed increases in theresistance, and increasing the feed rate and reducing the torque outputof said feed mechanism in response to all sensed decreases in theresistance there by constantly controlling the feeding of said elementin accordance with the changing condition of the workpiece.

8. In an abrading machine, the combination of, a tool having an abradingelement, mechanism for feeding said element relative to a workpiece ina'brading engagement therewith, a variable speed and torque electricmotor for driving said feeding mechanism at different rates determinedby the speed of operation of said motor, said feeding mechanismincluding reduction gearing between said motor and said element, and acontrol for said motor including a resistor connected in series with thearmature of the motor, said resistor having a current-versus-raistancecharacteristic that varies non-linearly with the resistance, increasingand decreasing with, and at a greater rate than increases and decreasesin the current through the resistor, thereby to adjust the terminalvoltage of said armature automatically in response to and in accordancewith variations in the resistance to feeding of said element relative tothe workpiece.

9. The combination as defined in claim 8 in which said resistor has apositive temperature coefficient and generates heat to change its owntemperature in proportion to changes in the current through theresistor.

10. The combination defined in claim 9 in which said resistor is anincandescent lamp filament in a protective atmosphere.

11. In a machine tool, the combination of, a tool element for performinga metal removing operation, mechanism for feeding said element intoengagement with a workpiece, a variable speed and torque electric motorfor driving said feeding mechanism at ditferent rates determined by thespeed of operation of said motor, and means sensing changes in theresistance to feeding of said element into the workpiece and operableautomatically in response to increases in the resistance to reduce thespeed of feed and control the torque output of said motor in accordancewith the changed resistance, said sensing means also operating inresponse to decreases in the resistance to increase the speed of feed ofsaid motor thereby to regulate the speed and torque in relation to theload encountered.

12. In an abrading machine, the combination of, a tool having a movableabrading element, mechanism for feeding said element into engagementwith a Work surface, a variable speed and torque electric motor havingan armature and driving said feeding mechanism to feed said element intosaid surface at diiferent rates determined by the speed of operation ofsaid motor, and a control for said motor including means responsiveautomatically to increases in the current drawn by said armature andoperating to reduce the terminal voltage of said motor progressively assaid current increases and at a rate greater than the rate of increasein the current, said means including a non-linear resistor connected inseries with said armature.

References Cited UNITED STATES PATENTS LESTER M. SWINGLE, PrimaryExaminer.

